The Terrors struggled throughout with their midfield still feeling the effects of losing Willo Flood and Scott Robertson.
But most worrying for manager Craig Levein must be the impotence of his strikeforce.
After an opening in which United had dominated possession, the visitors grabbed the lead with their first attack in the 16th minute courtesy of some dreadful defending.
Once again, it was a goal gifted in a very similar manner to their match last week at Hamilton.
It was a tame cross which should have been taken by Lukasz Zaluska but once again the keeper, as he done in recent months, was glued to his line leaving his indecisive defenders to fail to jump and give ex-United man Eric Odhiambo an easy finish from four yards out.
As United began to re-establish themselves, they had a strong claim for a penalty turned down in the 33rd minute when Lionel Djebi-Zadi pulled down Fran Sandaza on the six-yard line.
But it should have been two for Thistle three minutes from the break when Paul Dixon fluffed his clearance, allowing Doug Imrie to run in on goal.
With only the keeper to beat and Odhiambo to his left, the midfielder got caught up in indecision and the ball astonishingly trundled into the arms of Zaluska.
The Tangerines really should have equalised one minute after the break but off-form Jon Daly went down to ground too easily and spurned an easy opportunity.
But the visitors kept coming back and Imrie almost doubled the lead in the 58th minute after crashing a 12-yard shot into the side-netting.
The visitors were reduced to ten men in the 70th minute when sub Danny Swanson was sent through by Craig Conway only for Russell Duncan to ruthlessly bring him down and, as last man, he was sent packing.
With Inverness desperately hanging on, United's equaliser finally came in the 78th minute with a goal from Lee Wilkie.
A cross from Swanson was headed on by sub Andis Shala for the defender to ghost in behind the defence and flick the ball past Ryan Esson.